1. Field of the Invention
The invention relates to a switching stage having the following characteristics:
(a) a differential amplifier with a first and a second amplifier branch;
(b) the first amplifier branch has a resistor;
(c) a first terminal of the resistor is a terminal for a reference potential; and
(d) a second terminal of the resistor is connected to an output terminal (8) of the line driver switching stage through the base-to-emitter path of an emitter follower transistor.
One such switching stage is shown in the literature, namely in a article by George R. Watson, entitled "Advances in Bipolar VLSI" in Proceedings of the IEEE, Vol 78, No 11, November, 1990. Typically, the output of the switching stage is connected to a line on a printed wiring board. The characteristic wave impedance on the line is 50 ohms, as a standard. The line is terminated at the end by a wave resistor.
In order to keep the line at a high level (H), which for a reference potential of 0 V is typically on the order of -0.9 V, a static current of 22 mA flows through the emitter follower transistor, the line and the terminating resistor. Taking a finite current amplification of the emitter follower transistor into account, that current dictates a maximum resistance (200 ohms) for the load resistor in the differential amplifier branch. At a low level (L), which is typically on the order of -1.7 V, the switching current of the differential amplifier flows through the load resistor, so that a level rise of 0.8 V drops there. Thus if the maximum resistance of the load resistor is taken into account, a minimum value of 4 mA for the switching current is defined. The switching current and the current flowing through the emitter follower result in a relatively high power loss.